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== Latest comic ==

Revision as of 01:59, 30 October 2013

Welcome to the explain xkcd wiki! We have an explanation for all 2 xkcd comics, and only 0 (0%) are incomplete. Help us finish them!

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Timeline of the Universe
Not actual size, except technically at one spot near the left.
Title text: Not actual size, except technically at one spot near the left.

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: Created by the BIG BANG. Should include a list of the events, their times, and if they're real, explain what they are, and if they're jokes, explain what they are. Do NOT delete this tag too soon.

This comic is about the size of the visible universe, presented as a timeline in a way typical of representations of the timeline of epochs in cosmology.

Some events it describes, including the Big Bang and Inflation are real, but others are jokes, including the Medium Bang and Settling. The size history of the visible universe is also embellished for the sake of jokes; the actual size history of the universe has one period referred to as Inflation, which occurred shortly after the Big Bang, followed by comparatively gentle but accelerating expansion. This is artistically depicted in this image from NASA. Part of the humor in this comic comes from the fact that the varied rate of change in expansion is not yet fully understood, with explanations of events leading to this change including theories such as "dark matter" and "dark energy" (this might therefore be construed as "dark humour"). At the end of the drawing four possible continuations of the timeline are suggested, with director J. J. Abrams listed as the deciding factor between them, stating that all future cosmological development has been handed over to him. Abrams directed the Star Trek movie entitled Star Trek, which established additional alternate timelines for Star Trek, so it may be implied that multiple timelines could result from direction by Abrams in the future. Notably, each Star Trek series has included multiple interacting timelines. For information about each of the events shown in this comic's Timeline of the Universe, see detailed explanations in the section Events on the Timeline of the Universe below.

The title text is a variation of one of Randall's standard jokes that his drawings are Not actual size; in the case of this comic there is technically one spot near the left where the drawing depicts the actual relative size of the universe at the time the drawing represents. Where his drawing begins, at the time when the universe began, per definition, our visible universe had no measurable size. Very soon (within a tiny fraction of an attosecond) after the universe as we know it began, the inflation period blew it up very very fast and then it continued to expand until present day. So at some "time" after the big bang, our visible universe would have had a size (i.e. diameter) that would be the same as any thickness of Randall's universe "line". Since the universe as depicted in the comic goes from infinitesimal size at the moment of the Big Bang to the full size of the universe today, at some point near the left there will be a point where Randall's representation would have the same size as the universe at the correct "time period". Of course a problem with this is that there was only a very very short time period after inflation where the diameter of the observable universe is on the same scale as this comic, and that point is neither indicated nor likely to be accurate in relation to the duration of time elapsed. According to an answer given here regarding the size of the visible universe after inflation, there is reason to believe that the size was still less than 1 mm in diameter when the stage of expansion known as Inflation ended, which is less than the thickness of the line shown at the Big Bang (depending on the screen size the comic is viewed upon); So the point along the timeline where the size of our visible universe matched the line width appears after the Inflation period is thought to have ended. Since Randall includes the Medium Bang before Inflation on his drawing he has already inserted a mistake there, but as the next three epochs after Inflation are real epochs, it is likely somewhere in this part of the drawing that the visible universe would have had the same diameter as the thickness of the drawing at a relevant time epoch on the drawing. This will thus not be that far to the left but around the Quark epoch.

Events on the Timeline of the Universe

The events presented in the timeline are:

  • ?? (more than 13.8 billion years ago (Gya)): the unknown state of the universe prior to the Big Bang, if such a statement is even sensible. There are theories that our Universe is a bubble where inflation stopped (13.8 billion years ago in this universe) in an infinite and eternally inflating larger universe, which would give rise to the possibility of a multiverse with many bubble universes like ours where inflation has stopped. See for instance this recent video on the subject: How Many Universes Are There? from PBS Space Time. If this is true, the universe did not start at the big bang, but our part of the infinite universe actually began when the inflation period stopped, and not at the Big Bang.
  • Big Bang (13.8 Gya): The model of the origin of the universe which has achieved consensus among astronomers. We have observed that all galaxies are receding away from Earth at rates that are roughly proportional to their distance, and the simplest explanation for this is that the universe is expanding. If the universe is expanding, then (unless new physics are discovered) it must have at one time been very, very small and dense; that moment in time is called the Big Bang.
  • Planck Epoch: The time period starting from the Big Bang, the Planck epoch or Planck era is the earliest stage of expansion currently calculable, before the time passed was equal to the Planck time (tP, or approximately 10^-43 seconds). There is no currently available physical theory to describe such short times, and it is not clear in what sense the concept of time is meaningful for values smaller than the Planck time.
  • Medium Bang (a joke): If there's a Big Bang, why not have a medium one? There should probably also be a Little Bang, but maybe it's just too little to be featured on this chart.
  • Inflation (10^-36 to 10^-32 seconds after the Big Bang): A theory developed to explain the large-scale structure of the universe that postulates a period when the universe expanded very much faster than the speed of light. (The universe still expands faster than the speed of light, but only 2-3 times as fast. The limit of the speed c, is only valid for things moving in space time, not for the stretching of space itself!)
  • Quark Epoch (10^-12 seconds after the Big Bang): The universe is a quark-gluon plasma, up until 10^-6 seconds when it cools enough to coalesce into hadrons, including protons and neutrons.
  • Lepton Epoch (1 second after the Big Bang): Leptons, including electrons, and their associated neutrinos dominate.
  • Photon Epoch (10 seconds after the Big Bang): The universe is dominated by photons.
  • Cool Bug Epoch (a joke): There was a period around 10-17 million years after the Big Bang in which the cosmic background radiation was between 273 and 373 K, the temperature range for liquid water, but as oxygen had not yet been formed, as stars were not yet there to create it, there would have been no water. Cosmologists have speculated that primitive life could have arisen during this period and dubbed it the 'Habitable Epoch of the Early Universe', although it's unclear how this life would have formed since there was basically only hydrogen and helium atoms in the universe until the first Super Nova explosions some 100 million years later.
  • Molded grip (a joke): Some tools (e.g. knives) have molded finger-wells so that the user's hand settles easily and securely into a comfortable position. This epoch of the universe features repeated expansions and contractions so that this part of the timeline resembles a molded grip, at least in profile (it would be much too large to be held by any known animal's hands[citation needed], and the finger-wells are distributed over time as well as space).
  • Stars form (100 million years after the Big Bang): The universe cools enough to allow ordinary matter particles to group into stars.
  • Stagflation (a joke): In addition to cosmic inflation, inflation can also refer to the economic phenomenon in which prices increase over time. Stagflation is combination of the terms "stagnation" and "inflation", and refers to a situation in which monetary inflation is high, economic growth is slow, and job creation is low. This epoch of the universe shows the universe beginning to contract in size, much as economists would talk about an economy contracting.
  • Settling (a joke): Thanks to government intervention and quantitative easing of the cosmological constant, or perhaps the judicious use of the Universe Control Panel, the contraction of the universe has halted.
  • Rebound (a joke): Consumer confidence has returned to the universe and it has begun expanding again.
  • Someone tripped and accidentally hit the "Inflation" switch again (a joke). This switch must be on the Universe Control panel referenced both in 1620: Christmas Settings and in 1763: Catcalling.
  • Emergency Stop triggered (a joke). Also on the Universe Control panel see previous entry.
  • Galaxies form (12.8 Gya)
  • Earth forms (4.5 Gya)
  • Present day
  • Future cosmological development handed over to J.J. Abrams, outcome unknown (a joke): J.J. Abrams is a science-fiction writer and filmmaker. If he were in charge of the future development of the cosmos, he might decide to subject all of us to some strange plot twist. Among many other movies, he has directed the 2009 reboot of Star Trek, in which the "future history" of Star Trek is altered from the timeline of the original series by Nero and Spock traveling backwards in time. He also has directed other "Star" films, including Star Wars: The Force Awakens, and Star Wars: The Rise of Skywalker (to be released a few weeks after the publishing of this comic). He is also involved in the Mission: Impossible films.
    • The dashed lines coming off the end of the timeline represent the possible fates of the universe:
      • The one curving in represents that the universe could stop expanding and begin contracting, resulting in the Big Crunch. In our universe, cosmological measurements have shown that the expansion of the universe is accelerating, so the Big Crunch is considered to be the least likely fate.
      • The second curve continuing the trend from before represents that the universe could settle into thermal equilibrium, which would leave no energy available for any interesting phenomena to occur. This is called the heat death of the universe or "Big Freeze".
      • The fourth curve represents that the universe's expansion will continue accelerating, eventually very rapidly, to the point that the accelerating expansion overcomes all forces between particles, turning the universe into a collection of particles isolated from each other by rapidly-expanding space. This is called the Big Rip.
      • In between the second and fourth curve there seems to be something in between where the universe expansion accelerates and then slows down again. We have so far seen the expansion rate decelerate in the early life of the universe where the gravity of the more compact universe slowed the expansion, but then this turned around to an acceleration after about 9 billion years as the distance between galaxy clusters became so large that dark energy became the dominant force, causing the universe expansion to accelerate. So who knows it this could change again... At present our understanding says that the universe expansion-rate will keep accelerating. But left to J. J. Abrams, then the outcome is unknown. These four options represents both what we have theories for and what J. J. might come up with.

Transcript

[A large header is above the main drawing:]
Timeline of the Universe
[The drawing shows a diagram of the evolution of the universe from the Big Bang (left) to the present right with lines indicating possible futures continuing right of the main drawing. The drawing is that of a black horn of plenty, very thin to the left and then it becomes broader, mainly in steps but also slightly between each step. n a few places the diameter decreases a bit. Along the "tube" are segments divided with thin white lines, with about equal distance between them. The first 20 the tube is black, but then stars form, shown as many white dots, and finally in the last 3-4 segments galaxies are shown. At the top and bottom of the opening to the right there are four dashed lines which behaves the same way. Two points inward, two continue the trend from before they begin, two goes out fast again, and then falls back to slow increase, and two goes almost straight up and down. At the far left there is a line going in to a small dot. On the line before the dot are two questions mark. A line points to the dot which has a starburst around it. It represents the Big Bang. After this firs labeled point on the drawing there are mange other labeled sections with a line going from the label to a segment on the drawing. There are 9 above, 9 below and one at each end. The one at the right end pointing to the four dashed future lines at the top. From left to right in the order they are labeled on the timeline, the labels are:]
??
Big Bang
[The Universe comes in as a circle with action lines around it. It stays the same size for a while.]
Planck Epoch
Medium Bang
[The Universe starts inflating very slowly]
Inflation
[The Universe briefly inflates very rapidly, and returns to its normal rate of expansion.]
Quark Epoch
Lepton Epoch
Photon Epoch
Cool Bug Epoch
[The Universe starts inflating and deflating rapidly, as if to form a grip.]
Molded grip
[Stars appear in the timeline. The Universe starts inflating slightly faster than before.]
Stars form
Stagflation
[The Universe starts deflating slowly.]
Settling
Rebound
[The Universe starts inflating slowly again.]
Someone tripped and accidentally hit the "Inflation" switch again
[The Universe starts inflating at the same rate as the Inflation section.]
Emergency Stop triggered
[The Universe abruptly stops inflating, and stays level.]
[Galaxies appear in the timeline. The Universe starts inflating at a medium pace.]
Galaxies form
Earth forms
Present day
[We see the edge of the Universe, with a rounded shape. Various dotted line predictions are on the edges.]
Future Cosmological development handed over to J.J. Abrams, outcome unknown


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